As is known, in a paper making machine, a layer of cellulosic pulp, composed of approximately 3% fibre and mineral additives and approximately 97% water, is fed along a paper production line.
This production line goes through, in sequence, a cellulosic pulp formation and drainage section, a pressing section which produces a sheet of paper and a paper sheet drying section.
A first portion of the production line, which goes through the formation and drainage section, defines a drainage path, along which the cellulosic pulp is fed while supported by a fabric, that rotates in a loop, generally known as a “forming fabric”. Underneath the forming fabric, and in contact with the reverse side of the same fabric, is arranged, at a predetermined distance from one another, a number of blades (generally known as “foils”) and a number of suction units. The blades and the suction units are apt to remove the water contained between the fibres of the cellulosic pulp and which passes through the forming fabric as it advances. In particular, the blades remove the water that drains from the forming fabric by mechanical removal, whereas the suction units eliminate the water by the application of a vacuum.
A highly efficient formation and drainage section reduces the cost of pressing and drying operations carried out downstream of the formation and drainage section.
Portable devices are commercially available which monitor the efficiency of the formation and drainage station, by generally measuring the thickness of the cellulosic pulp layer or the amount of water contained in the cellulosic pulp.
Such devices generally include a rod on which a reading head is mounted and provided with a sensor, which is manually positioned in contact with the lower surface of the fabric between one suction unit and another, so as to measure the thickness of the cellulosic pulp layer positioned above the forming fabric.
Such devices employ various types of sensors, for example sensors that utilize GBS (Gamma Back Scattering) technology to measure the consistency of the material they contact. Such technology is accurate, but requires a radioactive source inside the reading head which makes it expensive and of no practical use due to the drawbacks of radioactivity. Other sensors utilize ultrasound and even if they are less expensive than the previous type, are difficult to use and are characterized by poor accuracy, especially in the environment of paper making machines.
Portable devices that utilize microwave sensors are also known, which estimate the quantity of water contained in the material by measuring the frequency response of the material. Such sensors are equipped with a resonance chamber made of very expensive metal-based alloy materials, since it is necessary to minimize the thermal expansion or contraction effects. In such sensors, in fact, the thermal expansion or contraction causes a shift in the resonance frequency which affects the response and, therefore, the accuracy of the measurement of the sensor. In addition, the field of microwaves output by sensors of this type has a poorly defined shape and a limited penetration capacity inside the layer of material. Hence such sensors cannot be used with very thick layers of material. Finally, the resonance chamber of such microwave sensors have minimum dimensions that do not allow the sensor to be integrated inside the blades of the formation and drainage section.